1. Field of the Invention
This invention relates to amplifiers in general, and, more particularly, to wide-bandwidth operational amplifiers.
2. Description of the Prior Art
Operational amplifiers are the building blocks of many complex functions, such as analog-to-digital (A/D) and digital-to-analog (D/A) converters, instrumentation amplifiers, voltage regulators, etc. The operational amplifier (op amp) is commonly understood to have a differential input and a single-ended output. Ideally, the differential mode voltage gain (the amplification of a signal applied substantially between the differential inputs) of an op amp is infinite and common mode voltage gain (the amplification of a signal referenced to ground and applied to the differential inputs equally) is zero. Further, the bandwidth of an ideal op amp is infinite. Unfortunately, realizable op amps fall very short of this ideal. One consequence of less than infinite differential mode voltage gain and limited bandwidth is the distortion of signals passing through the op amp.
A conventional op amp 101, used as anon-inverting amplifier 100, is shown in FIG. 3. The input to the amplifier 100 is designated V.sub.IN and the output is designated V.sub.OUT. It is noted that the designations of the input and output of the amplifier 100 also refers to the voltage of signals thereon. The op amp 101 is powered by voltages V.sup.30 and V.sup.-. The differential inputs, labeled + and - for the non-inverting and inverting inputs to op amp 101, respectively, couple to the input of the amplifier 100, V.sub.IN, and to a feedback network of resistors 102, 103. Resistor 103, coupling to the output of the op amp 101, feeds back the voltage therefrom, attenuated by the voltage divider formed by the resistors 102, 103, to the inverting input to the op amp 101. Using R.sub.102 and R.sub.103 to refer to the resistances of resistors 102, 103, respectively, the gain (loss) of the voltage divider, K, is: ##EQU1## and the approximate closed-loop voltage gain of the amplifier 100 can be expressed as: ##EQU2## where A.sub.d is the differential mode voltage gain of the op amp 101 and the common mode voltage gain, if any, is neglected. If the op amp 101 is an ideal op amp, i.e., A.sub.d =.infin., the gain of the amplifier 100 is 1/K. However, when using a realizable op amp for the op amp 101, at low frequencies, the overall voltage gain of the amplifier 100 is not quite 1/K due to the finite differential mode voltage gain (A.sub.d). Still further, the gain of the amplifier 100 decreases from 1/K with increasing input signal frequency (applied to V.sub.IN). The reason for this is illustrated in FIG. 4 and discussed below.
In FIG. 4, a simplified gain (in dB) versus frequency plot (not to scale) of a realizable op amp is shown. Curve 110 corresponds to the differential voltage gain, A.sub.d, of the op amp 101 (FIG. 1). Note that the differential gain, A.sub.d, at low frequencies (zero Hz through approximately f.sub.OL) is very high, typically 80 to 100 dB, and drops off until the gain reach 0 dB (unity gain) at the frequency f.sub.m, commonly referred to as the gain-bandwidth product frequency. The gain-bandwidth product is a figure of merit used to measure the performance of the op amp. The f.sub.OL, or open-loop dominant pole frequency, results from a dominant pole (typically a capacitor internal to the op amp 101, not shown) introduced into the op amp 101 to insure stability of the amplifier 100 (FIG. 3) due to the high differential gain of the op amp 101.
As discussed above, the overall voltage gain of the amplifier 100 deviates from 1/K when the frequency of the input signal is increased beyond f.sub.OL. As can be demonstrated from above equation for the closed-loop voltage gain of the amplifier 100, when the differential voltage gain of the op amp 101 decreases, the gain of the amplifier 100 decreases from 1/K. This is illustrated by the dashed portion of the curve 111 shown deviating from the desired curve (solid line of curve 111). This decrease in gain in a source of distortion to wide bandwidth signals amplified by the amplifier 100. For example, pulses or high-fidelity audio signals amplified by the amplifier 100, will be rounded off and distorted. Further, voltage regulators, using amplifier 100 to regulate an output voltage, will not respond to load changes to the regulator as quickly as desired, requiring multiple and/or large high-frequency bypass capacitors on the output of the regulator. Hence, it is desirable to provide an op amp having wide bandwidth to minimize the distortion of signals amplified by the op amp without compromising the stability of the amplifier utilizing said op amp.